Field of the Invention
The invention relates to an electrohydraulic regulating device, particularly for a motor vehicle rear axle steering assembly, having an electrically actuated control valve by which a hydraulic fluid flow to an adjusting cylinder is controlled; at least one electrically actuated switching valve by which a hydraulic fluid flow is blocked or admitted; and a control unit by which control signals for the control valve and the switching valve are generated.
Such regulating devices are, for instance, used in rear axle steering assemblies of motor vehicles, but may also be used in automatic motor vehicle transmissions, for example.
One rear axle steering assembly which is known from U.S. Pat. No. 4,770,264 includes such a regulating device, which has a hydraulic power part and an electronic signal processing part. The power part includes an electrically actuated control valve, by means of which a flow of hydraulic fluid to an adjusting cylinder is regulated, and a plurality of electric switching valves, by which the fluid flows are blocked off or admitted. The electronic signal processing part includes a microcomputer-controlled control unit, by which control signals for the control valve and for the switching valve or valves are generated. An electronic control unit having a task of setting the position of the adjusting cylinder piston with high accuracy even if interfering suspension forces at the rear axle act on the regulating system, is known from Published International Application WO 89/10865, for instance.
One example of an electrically actuated control valve is a proportional valve, which furnishes a continually varying output variable (such as a volumetric oil flow or a pressure) as a function of a continuously variable input variable (such as an electric current). The task of a switching valve is either to block off a volumetric oil flow entirely, or to admit it with the least possible pressure losses. In order to describe the electrical properties of a switching valve, a switching current and a holding current are typically defined. At least the switching current must flow in the coil of the switching magnet (which is also called a valve coil), so that an actual modulation of the hydraulic actuating elements will ensue. If the modulated state is to be held, then at least the holding current must be flowing in the valve coil.
Simple end stages for switching valves typically have only a purely yes/no function (switched/not switched). Since the supply voltage of the switching valve and the valve temperature and therefore the valve resistance directly and definitively influence the level of electric current in the valve, the valve and the associated power end stages have previously been constructed in such a way that the various switching and holding currents will still be reliably attained even at maximum operating temperature and minimum valve operating voltage. As a consequence of that construction, in all other cases (such as lower operating temperature, higher operating voltage), the current in the switching magnet coil of the valve is higher than necessary. Therefore, an unnecessarily high power loss is produced in the valve, end stage and control unit.
It is accordingly an object of the invention to provide an electrohydraulic regulating device, particularly for a motor vehicle rear axle steering assembly, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which lowers the power loss in electrohydraulic regulating devices.